Molluscs of an intertidal soft-sediment area in China: Does overfishing explain a high density but low diversity community that benefits staging shorebirds?

• Published in: Journal of Sea Research
• Main Authors: Yang, H-Y, Chen, B., Piersma, T., Zhang, Z., Ding, C.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2016
• Subjects: Arctic; Calidris canutus
• Online Access: http://imis.nioz.nl/imis.php?module=ref&refid=254556

The YellowSea is a key staging ground for shorebirds that migrate fromAustralasia to the Arctic each spring. A lotof attention has been paid to the impact of habitat loss due to land reclamation on shorebird survival, but any effectsof overfishing of coastal resources are unclear. In this study, the abundance of molluscs in the intertidalmudflats of northern Bohai Bay on the Chinese Yellow Sea was investigated in 2008–2014 from the perspectiveof their importance as food for northwardmigrating shorebirds, especially Red Knots Calidris canutus. Numericallycontributing 96% to the numbers of 17 species found in spring 2008, the bivalve Potamocorbula laevis (the staplefood of Red Knots and other shorebirds) dominated the intertidal mollusc community. In the spring of 2008–2014, the densities of P. laevis were surprisingly high, varying between 3900 and 41,000 individuals/m2 at distinctlysmall sizes (average shell lengths of 1.1 to 4.8mm), and thus reaching some of the highest densities ofmarinebivalves recorded worldwide and providing good food for shorebirds. The distribution of P. laevis wasassociated with relatively soft sediments in close proximity to the recently built seawalls. A monthly samplingprogramme showed steep seasonal changes in abundance and size. P. laevis were nearly absent in winter, eachyear settling on the intertidal mudflats anew. Peak densities were reached in spring, when 0-age P. laevis were1–3 mm long. The findings point to a highly unusual demographic structure of the species, suggesting thatsome interfering factors are at play. We hypothesise that the current dominance of young P. laevis in Bohai Bayreflects the combined pressures of a nearly complete active removal of adult populations from mid-summer toautumn for shrimp farming (this clearing of adults may offer space for recruitment during the next spring)and low numbers of epibenthic predators of bivalves, such as shrimps and crabs, due to persistent overfishingin recent decades (allowing freshly settled juveniles to reach high densities). To the best of our knowledge, theidea that overfishing of competing marine mesopredators benefits staging shorebirds, at least in the shortterm, is novel; it nowneeds further experimental and comparative scrutiny. The long-termeffects of overfishingon benthic communities of the mudflats need further investigation.